The present invention relates to probes used for testing electrical circuits and, more particularly, to a probe for providing electrical contact to electrical circuits during the testing thereof comprising, tubular housing having an open end and a generally closed end having a small cleaning hole therethrough; a probe plunger disposed in the tubular housing, the probe plunger comprising a cylindrical inner portion being a slide fit within the tubular housing and a cylindrical outer portion of a diameter smaller than the inner portion and forming a shoulder with the inner portion at a point of meeting; a longitudinally compressed bias spring disposed between the closed end of the tubular housing and the inner portion of the probe plunger for urging the probe plunger to an extended position; and, a cylindrical retaining and sliding bearing region formed adjacent the open end of the tubular housing and having the outer portion of the plunger passing therethrough, the retaining and sliding bearing region having an internal diameter which is a slide fit to the outside diameter of the outer portion.
Test probes are used in groups of hundreds or thousands for contacting a printed circuit board under test. Computer-based testers are used to supply signals and analyze outputs from the board under test. One hundred percent production testing of computer boards and other electronic equipment board is the typical industry standard.
For such tests to be efficient and accurate, the probes employed must be very reliable. In prior art test probes, there are several points of probe failure. One is the failure of the probe's tip to pierce the flux or oxide films on the solder-coated pads of the circuit board. This problem typically applies to in-circuit testing after components are assembled to the board and the board itself has been wave soldered, or the like, to affix the components to the board. Another major point of probe failure is a result of internal wear of the probe itself, which generates contaminating particulates. These particulates, in turn, prevent good electrical contact between the probe plunger and the tubular housing. This wear can be caused by spring fatigue (which allows sharp broken coils to scrape particles from the tubular housing's internal surfaces) or as a result of probe side loading. Side loading occurs if probe points engage the sloped sides of solder lumps or bent component leads, or if single pointed probes enter misaligned holes in test boards.
The probe generally indicated as 10 in FIG. 1 represents an early design in the art. It was manufactured in strokes up to 0.16 inches for 0.1 inch mounting centers. The probe plunger 12 comprises an inner portion 14 which has a diameter sized to be a slide fit in the tubular housing 16. An outer portion 18 extends outward concentrically from the inner portion 14. The outer portion 18 is of a smaller diameter than the inner portion 14 thereby forming a shoulder 20 at their point of joining. The probe plunger 12 is retained in the tubular housing 16 by crimping the housing around the outer portion 18. As depicted in FIG. 2, the tolerances must be kept quite close to prevent problems. If the tolerance between the outside diameter of the inner portion 14 is not very close with respect to the inner diameter of the tubular housing 16, the plunger 12 scrapes the sharp edge of the crimp, as indicated by the arrow 22, generating wear particles which rapidly degrade the performance of the probe 10. If the tolerance is too close, the plunger 12 can bind in the housing 16, causing probe failure. Likewise, excessive side loading can cause rubbing and wear regardless of the tolerances held during assembly. Also, as mentioned above and as indicated by the arrow 24, breaking of the outward bias spring 26 can cause sharp edges that produce undesired and damaging particles through scraping.
Industry testing requirements developed in the late 1970's created a demand for longer stroke probes; that is, the distance the tip 28 carried by the end of the outer portion 18 can move inward towards the housing 16. In response to these requirements, the industry soon adopted the probe configuration of FIG. 3, which resolved the wear problems of the prior art probes of FIGS. 1 and 2 while offering the required longer stroke. The probe 10' of FIG. 3 remains the most common configuration throughout the industry to date. Examples can be seen in U.S. Letters Pat. No. 4,397,519 of Cooney and U.S. Letters Pat. No. 4,659,987 of Coe et al., the latter patent being assigned to the common assignee of this application.
While solving the wear problem and offering a longer stroke (relatively speaking as compared to the prior art), a continuing problem with the prior art probe design of FIG. 3 is that the evolved standard industry dimensions for the equipment in which it is employed do not permit adequate space for a long life spring which will also provide enough force to reliably pierce board contaminants as described above. Long spring fatigue life becomes particularly important in universal test beds. Such beds use 100 test probes per square inch and frequently use 30 or 40 thousand probes per bed. In this style of tester, the same probes are used over and over again for all the types of boards tested. Probes in such a tester will often encounter 250,000 test cycles per year. Spring breakage is a serious concern using the probes 10' of FIG. 3.
Another problem with the prior art probe 10' of FIG. 3 is that it is unstable during assembly by the automated apparatus employed for the purpose. Note that the probe plunger 12' of this design has an inner portion 14' and an outer portion 18' of the same diameter separated by a middle portion 30 of reduced diameter. The plunger 12' is retained within the housing 16 by a crimp 32 disposed along the length of the housing 16 at a point which will intercept the shoulder 34 formed by the junction of the inner portion 14' and the middle portion 30. The plunger 12' is guided longitudinally within the housing 16 by the combined effect of the outer portion 18' and inner portion 14' sliding within the tubular housing 16. As can be appreciated by those skilled in the art, however, during assembly the outer portion 18' must fit within the housing 16 a sufficient distance following insertion at one station so as to remain stable during movement to the next station when the plunger can be depressed to pre-load the spring 26 and the crimp 32 made in the housing 16. This forces the spring 26 to be shorter than would be desirable. In order to get the required piercing force at the tip, the spring 26 must, therefore, be quite stiff. This results in a higher propensity to breakage of the spring 26. The prior art probe 10' of FIG. 3 typically provides full stroke spring life below 100,000 cycles. As can be appreciated, in apparatus such as the test bed described above performing 250,000 cycles per year, this means that each of the 30 or 40 thousand probes in the bed only has a spring life expectancy of 4.8 months--following which, the formation of particles through scraping and deterioration of probe performance begins.
Wherefore, it is an object of the present invention to provide a test probe for use in electrical component test beds, and the like, which has a design which resists scraping action between the components which can produce probe-deteriorating particles.
It is another object of the present invention to provide a test probe for use in electrical component test beds, and the like, which has a design which provides increased shut height for a given spring length.
It is still another object of the present invention to provide a test probe for use in electrical component test beds, and the like, which provides increased spring life expectancy without breaking.
Other objects and benefits of the present invention will become apparent from the description which follows hereinafter when taken in conjunction with the drawing figures which accompany it.